This is the second most popular page on our website. Interestingly, more than half the viewers searched using "500 SF/Ton" in the search phrase. Why is that interesting, you ask? Because very few homes require as much cooling as 500 SF/Ton yields. In fact, ACCA has only identified three conditions where 500 SF/Ton MIGHT be required:1. A home with inefficient construction (old windows, substandard insulation, high air leakage to/from outdoors, etc.).2. A home with a substandard duct system in an unconditioned space.3. A home with a lot of windows (more than 30% of wall area) exposed to direct sunlight.

Those homeowners who aren't in that group can think in terms of 700 SF/Ton and up - to as much as 1,200 SF/Ton.

Those homeowners who are in that group should first look into ways to get out, before spending money on additional capacity:1. Improving the inefficient construction (scroll down to learn about the "Blower Door Test").2. Fixing that leaky duct system (ACCA tells us that can reduce a home's cooling requirement by 100 to 400 SF/Ton!).3. Investing in shading devices.

We've just added our comments on load calculations submitted by a contractor. You might want to read them. Here is the home in question.

We can't remember seeing an undersized air conditioner in a residence: The jobs we see have too much cooling and too little ductwork; repairing the ductwork increases DELIVERED CAPACITY, so if you think your air conditioner is too small, always review your duct installation before increasing cooling capacity.

Basics

Your air conditioning must run for extended periods at peak times in hot, humid weather to properly dehumidify your home. You’re more comfortable in a dry, warm atmosphere than in a cool, moist atmosphere; and there’s much less risk of mold growth in a drier atmosphere. Those facts stress the need to properly size air conditioners.

IMPORTANT NOTE: When it comes to air conditioning, less is better, and you must resist suggestions to “go a little bigger” to be safe; the contractor is just following his "Business Plan" (which is to sell as many tons as he can).

When a furnace is provided, it must be selected for cooling airflow (furnaces are manufactured in various combinations of heating capacity and cooling airflow), so upsizing the cooling may mean unnecessarily upsizing the heating as well. By upsizing the cooling, then, you may be paying three ways: A larger air conditioner, larger ducts (more airflow, remember) and a larger furnace; and that’s only installed cost, wait till you start paying to operate the oversized equipment.

Oversizing furnaces jeopardizes heating comfort, and often generates the “blast of heat” syndrome: The furnace only runs for a short time, even in very cold weather; the resulting “swings” in indoor temperature are quite noticeable, and objectionable to many. As with air conditioning in hot weather, you want the furnace to run relatively continuously in very cold weather; here the issue is comfort, there is no mold risk.

TRUE STORY: We recently reviewed a home in Rehoboth Beach, DE in which the furnace was large enough to heat the home to -40F (Yes, that's 40 degrees F BELOW zero); since design there is +18F, that home won't be real comfortable in cold weather.

IMPORTANT NOTE: Service life for air conditioners and heat pumps is around 15 years; if it's time to replace an existing unit, you have a once in a lifetime opportunity to correct oversizing. Whatever you do, don't arbitrarily go bigger without first reviewing your duct system.

Advanced

How To Tell If Your Air Conditioner Is Too Big

Time the operation of your outdoor unit on a hot day (Ideally the temperature should be at least the "1% value" - see below). Say it runs 20 minutes, is off five minutes and then runs again. That means it runs 80% (20/25) of the time. If you have a 3 ton unit, you could safely reduce to 2-1/2 tons; if you haven't fixed your ductwork yet, you could probably reduce to 2 tons when you do.

" ASHRAE 1% Value"

We no longer design cooling systems for "Worst Case" temperatures; that's been illegal for decades. Today, we use the 1% value: That temperature that historic records indicate is exceeded 1% of the time (87.6 hours per 365 day year).

OH MY GOD, ARE YOU NUTS? IT GETS HOTTER THAN THAT - I'M SCARED!

Well, think about it: So it gets hotter; it doesn't stay hotter 24/7, and the sun has a way of moving around during the day and going away periodically; you can't hurt your equipment running it continuously for long periods of time, it won't get old and tired. What we like to see is relatively continuous operation at peak times during oppressively hot weather; we may see the indoor temperature "float" a degree or two. This is nowhere near having a heater that's too small and you're "FREEZING". Beware: Unscrupulous contractors prey on this type of groundless fear.

WHAT YOU NEED TO KNOW ABOUT "TEMPERATURE FLOAT"

It's July 21, 2011 and the outside is seething: 97 degrees dry bulb (temperature) and 79 degrees wet bulb (a measure of moisture). ASHRAE 1% design is 90/76, the AC's been running continuously for some time, and the indoor conditions are ranging around 76 degrees with a 75 degree setpoint, so there's some float. Humidity is in the mid to low 50's, there's continuous air movement, and it feels GOOD in here: None of that "clammy" sensation you often get with 500 SF/Ton (we're in excess of 700SF/Ton with the duct improvements and downsizing).

Our major "fenestration" (think windows) exposure is SW, which has just peaked in terms of "solar gain". That means the indoor temperature will soon go down to setpoint, and the AC will cycle off. In the interim, it's been very comfortable.

HELP! IT'S HOT AND MY AC'S NOT KEEPING UP!

We can't remember any homes with insufficient capacity; we do see beaucoup with TOO MUCH EQUIPMENT, TOO LITTLE AIRFLOW. Flexible duct workman-ship is a big problem, and if you have a "Flexible Duct System" you can forget about adequate airflow. Duct leakage can also be a big problem: Suppose you're drawing 130 degree air from your attic because the return ductwork is not sealed as required by Code; or suppose your supply ductwork is not sealed and you're air conditioning your attic. We also see a lot of undersized ductwork; can you imagine having to breathe through a drinking straw? Much of what we do involves reducing installed capacity (in conjunction with ductwork improvements, of course).

Time to Add on to Your Home?

There's good news: You probably already have enough cooling to support a 15% or 20% increase in floor space; you can use the money you would have spent for an additional system improving your old system (repairing ductwork, implementing zoning, giving the old 8 or 10 SEER the deep six, etc.); you may even end up buying less energy every year.

Time to Replace Your Old Equipment?

Here's more good news: You probably won't have to buy as much capacity as you now have; you can use the money you would have spent for the additional capacity improving your old system (repairing ductwork, implementing zoning, etc.). And now's the time to improve the building envelope: Consider a "Blower Door Test", improving your insulation, replacing windows, etc.

BETTER DESIGN SAVES MONEY - OR - 933 SF/Ton was fine.

We were commissioned to review a proposal for air conditioning a new home, and we called the builder to discuss his sub's bid. He volunteered his sub’s HVAC design procedure, and here it is in its entirety:1. Determine tonnage4,200 sf gross x 0.67 = 2,814 sf net2,814 sf net ÷ 400 sf/ton = 7½ tons (note that they rounded up, not down)Doing a little math, we see this contractor was using 597 square feet per ton, based on the gross footage.2. Determine number of supplies7½ tons x 400 cfm/ton = 3,000 cfm (that’s Cubic Feet per Minute, a measure of airflow)3,000 cfm ÷ 100 cfm each = 30 supplies required3. Determine number of returns8 rooms @ 1 each = 8 total4. Summary: 7½ tons, 30 supplies, 8 returns

After studying the plans and "Doing The Math", we recommended 4½ tons, 13 supplies and 5 returns. The homeowners saved about $4,000 on the installation, and pocketed the difference between that and our fee; they were quite comfortable during the ten years they lived there.

The design procedure the subcontractor used is good for business: It’s quick, there’s no need to worry about how big the windows are or what direction they face (or whether the lot is shaded or sunny, for that matter), there’s always enough cooling, and plenty of cooling equipment gets sold. Because the system's oversized, cycling a lot and running inefficiently, it'll need more service; that too is good for business.

IMPORTANT NOTE: The fact that the square foot method is illegal today doesn't seem to matter; there are still contractors out there using that as we speak. Watch out for those "Dinosaurs", they'll sell you more than you need in a "New York Minute".

IMPORTANT NOTE: It is prohibited to use floor space as the determining factor to select heating and cooling equipment; it can take many hours to properly calculate heating, cooling and room airflow requirements

BUYERS OF LUXURY HOMES BEWARE!

Many HVAC contractors only know one way to make more money: To sell more tons; it often seems that expensive homes have a way of ending up with more tons per SF than lesser homes, even though more expensive homes usually have the best windows money can buy, and are built better.

FYI: The cost per ton ($2,500 to $5,000 give or take) take) doesn't seem to vary for the same type of work done in different classes of home.

THE SQUARE FOOT METHOD

ACCA (Air Conditioning Contractors of America, developers of "Manual J") tells us "Efficient single family detached homes with a normal amount of well distributed glass typically fall in the 700 to 1,200 range" (para. 2-4, page 9). That's700 to 1,200 square feet per ton of cooling; we're not sure if the includes thicknesses of outside walls, closets, "air space" beyond balconies on upper floors, etc.

It's illegal to use square feet of floor space to determine heating and cooling requirements.

It's especially illegal to use 400 or 500 square feet per ton for every home.

Here's our plan to remove 22% of installed cooling capacity from a large luxury home: One unit was removed completely, another was downsized, and several were moved around.

This equipment manufacturer has the following clearance requirements (to facilitate airflow through the units, optimizing unit performance):48" above,6" on one side and 12" on the other, and 24" between adjacent units

Those requirements were not met, the ambient air was much hotter than it should have been, and that reduced cooling capacity; that belief drove our decision making.

We also specified two large fans to circulate cooler air through the space.

COMMON MANUAL J MISTAKES

Using other than the 1% value.

Not assuming there are drapes or blinds.

Not assuming drapes or blinds will be closed (an exception is permitted for daylighting).

Not evaluating shade from roof overhangs.

Not taking benefit of window screens.

Using excessively high infiltration rates (ACCA has accumulated a lot of data in this area).

Here's a"Blower Door Test" used to find and repair air leaks in the building envelope.

It's a good idea when building a home, or when considering replacing old equipment: Why buy more than you need?

More from ACCA on the Square Foot Method"Single family dwellings that are characterized by inefficient construction and/or inefficient duct system (in an unconditioned space) may be in the 500 to 700 SqFt/Ton range.""Single family detached dwellings that have reasonably efficient construction and a reasonably efficient duct system may be in the 700 to 1,000 SqFt/Ton range.""Efficient single family detached dwellings that have a normal amount of sunlit glass areas may be in the 800 to 1,200 SqFt/Ton range.""Efficient single family detached dwellings that have large sunlit glass areas may be in the 500 to 800 SqFt/Ton range."Manual J, page 73